In a mobile communication system which uses multiple carriers, such as an orthogonal frequency division multiple access (OFDMA) or a single carrier-frequency division multiple access (SC-FDMA), radio resources are a set of continuous sub-carriers and are defined by a time-frequency region on a two-dimensional sphere. A time-frequency region in the OFDM or OFDMA scheme is a rectangular form sectioned by time and sub-carrier coordinates. In other words, one time-frequency region could be a rectangular form sectioned by at least one symbol on a time axis and a plurality of sub-carriers on a frequency axis. Such a time-frequency region can be allocated to an uplink for a specific user equipment (UE), or a base station can transmit the time-frequency region to a specific user equipment in a downlink. In order to define such a time-frequency region on the two-dimensional sphere, the number of OFDM symbols on the time region and the number of continuous sub-carriers on the frequency region should be given, wherein the continuous sub-carriers start from a point having an offset from a reference point.
FIG. 1 illustrates an example of a structure of a physical channel used in a multiple carrier system according to the related art. In FIG. 1, a sub-frame comprises an L1/L2 control information transmission region (hatching part) and a data transmission region (non-hatching part).
Referring to FIG. 1, a physical channel includes a plurality of sub-frames on the time axis and a plurality of sub-carriers on the frequency axis, wherein one sub-frame includes a plurality of symbols on the time axis. One sub-frame includes a plurality of resource blocks (RBs), each of which includes a plurality of symbols and a plurality of sub-carriers. Also, each sub-frame can use specific sub-carriers of specific symbols (for example, first symbols) for a physical downlink control channel (PDCCH), i.e., L1/L2 control channel. One sub-frame has a length of 0.5 ms, and transmission time interval (TTI) that is a unit time of data transmission has a length of 1 ms corresponding to two sub-frames.
In a mobile communication system, radio resources of one cell includes uplink radio resources and downlink radio resources. The base station serves to allocate and control downlink radio resources and uplink radio resources of a cell. In other words, the base station determines what user equipment uses what kinds of radio resources and when the corresponding user equipment uses the corresponding radio resources. For example, the base station can determine that frequencies 100 Mhz and 101 Mhz are allocated to a first user equipment for downlink data transmission for 0.2 seconds after 3.2 seconds. The base station notifies the first user equipment of the determined fact so that the first user equipment can receive downlink data. Likewise, the base station determines what user equipment transmits uplink data through an uplink using how many radio resources, and also determines when the corresponding user equipment transmits uplink data. Also, the base station notifies the corresponding user equipment of the determined fact so that the corresponding user equipment can transmit uplink data.
In the related art, a specific user equipment has continued to use specific radio resources while call connection is being maintained. However, such a structure is inefficient in a recent communication system, which provides many services based on IP packets. This is because that most of packet services do not generate packets continuously for call connection time. Namely, packets may be transmitted for a specific interval but no packets may be transmitted for another specific interval. It is not efficient that radio resources are continuously allocated to a specific user equipment for call connection in the above packet-based system. In order to solve this problem, a recent mobile communication system uses a method of dynamically allocating radio resources to the user equipment if the user equipment needs the radio resources or only if there are data to be transmitted to the user equipment.
In the system which uses OFDM or SC-FDMA system, a frequency band is divided into bands of a constant size and each band is allocated to several user equipments. In this case, the base station may not receive data transmitted to an uplink through each frequency band due to interference of data transmitted from another band. In order to avoid this, synchronization in transmission time between the respective user equipments is necessarily required. In other words, when the first user equipment and the second user equipment are scheduled to transmit uplink data for a specific time interval, the time when the data transmitted from the first user equipment arrive in the base station should be identical with the time when the data transmitted from the second user equipment arrive in the base station. At this time, if there is a little difference in timings when the data transmitted from the first and second user equipments arrive in the base station, the data transmitted from the first and second user equipments cannot be recovered in the base station successfully.
Accordingly, in the system which uses OFDM or SC-FDMA system, uplink synchronization of the respective user equipments is necessarily required. To maintain uplink synchronization, various methods are used. One of the various methods is a synchronization method based on a random access procedure through a random access channel (RACH).
The random access procedure will now be described in brief. The user equipment, which is in a non-synchronization state, transmits a bit stream, which is previously determined, i.e., signature, to the base station. The base station detects the signature, and calculates how slow data transmission of the user equipment should be performed or how fast data transmission of the user equipment should be performed, so as to reach uplink synchronization based on the detected signal. The base station reports the calculated result to the user equipment. The user equipment adjusts the transmission time of uplink data based on the calculated result and then obtains uplink synchronization.
Hereinafter, radio resource control (RRC) state of the user equipment and its RRC connection method will be described. The RRC state means whether RRC layer of the user equipment is logically connected with RRC layer of the network. The RRC state is called RRC connected state if the RRC layer of the user equipment is logically connected with the RRC layer of the network. On the other hand, the RRC state is called RRC idle state if not so. The network can identify the presence of the user equipment of the RRC connected state in a cell unit due to the presence of the RRC connection. Accordingly, the network can effectively control the user equipment. By contrast, the network cannot identify the user equipment of the RRC idle state, and a core network manages the user equipment of the idle state in a unit of a tracking area which is a local unit greater than the cell. In other words, in case of the user equipment of the RRC idle state, its presence is identified in a great local unit. The user equipment should be shifted to the RRC connected state to obtain a typical mobile communication service such as voice or data.
When a user first turns on the power of the user equipment, the user equipment searches a proper cell and then stays in the corresponding cell in the RRC idle state. The user equipment, which stays in the RRC idle state, performs RRC connection with the RRC layer of the network through an RRC connection procedure if the RRC connection is required, and is shifted to the RRC connected state. The user equipment, which is in the RRC idle state, needs the RRC connection in case of various examples. For example, the user equipment which is in the RRC idle state needs the RRC connection if uplink data transmission is needed due to trying calling of the user or if a response message transmission to a paging message received from the network is needed.